There is hubris about Iranian behaviour that can be breathtaking. Far from expressing mock horror at Western accusations that Iran is funding Sunni as well as Shia extremists in Iraq, senior Iranian advisers nowadays go out of their way to make explicit their determination to harass and confront the Americans throughout the Middle East.

Describing US forces in Iraq as Iranian “hostages”, they brush aside threats of US military action against Iran as fanciful. They make no attempt to conceal their readiness to use proxies such as Hezbollah to stir up antiWestern feeling. And they admit that, despite deep antagonism with the Sunni Taleban, Iran is backing the Afghan insurgents simply in order to deny Nato a role in shaping the country’s future.

American accusations that Iran, and in particular its powerful Revolutionary Guards, is intent on destabilising the two countries and is supplying and training insurgents, are, if anything, an understatement: Gulf states regard Iran as a hegemonistic bully, ready to use force, terrorism and political subversion to dominate its neighbours and harness a resurgent Persian nationalism to the zealotry of its clerical hardliners.

The Gulf states therefore will welcome Washington’s announcement of new sanctions and the description of the Quds Force, the overseas operations arm of the Revolutionary Guards, as a “supporter of terrorism”. The pretext for this announcement, which Condoleezza Rice called part of a policy to confront the Iranians’ threatening behaviour, is the failure so far of the United Nations Security Council to mandate any tougher sanctions against Tehran. The real reason, however, is US anger with Iranian intransigence in the fruitless diplomatic talks on Iran’s nuclear programme and its frustration with the determination of Russia and China, for narrowly selfish motives, to block any sanctions with bite.

The new American measures are well targeted. The 125,000-strong Guards force was set up in parallel to the Iranian army to protect the clerical system after the 1979 revolution. But it has evolved into an untouchable elite that bolsters its power and wealth with sprawling business interests, ranging from construction work to oil and gas projects. President Ahmadinejad is himself a former Guard, and his political appointments, including provincial governors and more senior posts, have been from comrades in arms.

Iran has dismissed the sanctions as doomed to failure. It is in danger of overestimating its immunity. The high oil price has indeed bolstered national income, dampening for the moment general dissatisfaction among the have-nots and a frustrated business community with the corrupt clerical Establishment. But the tightening restrictions on Iran’s banking operations, on the sale of technology and on investment in the vital energy sector have begun to bite. Nor is belligerence the best remedy for a shaky economy.

Tehran also appears to underestimate the determination among some influential advisers in Washington to use a military option in a striking at Iran’s nuclear installations, despite European opposition and the warnings of US overstretch in Iraq and Afghanistan. Despite the silence of both Israel and Syria, the mysterious Israeli strike on a site believed to be a Syrian nuclear facility under construction has sharpened the view that pin-point air strikes can be effective and may not inevitably lead to a full-scale military engagement.

Washington has also been bolstered by the full backing President Sarkozy has given to a stronger line against Tehran. President Putin’s opposition to tougher sanctions has angered Europe, and has been a main bone of contention at the EU-Russia summit in Lisbon. But Moscow, which supplies much of the equipment vital to any Iranian nuclear programme, has a clear understanding of how far the programme has developed; its opposition to sanctions may be based more on a wish to deny the US the lead on an issue that Russia feels is one where it has a right to be the decisive voice. Counting on Moscow’s protection would be a grave mistake for Iran. The new US sanctions send a clear message, and one that Tehran and its new nuclear negotiation would be wise to heed.